1. Field of the Invention
The present invention relates generally to protection devices for loads, and more particularly to a thermal protection device for protecting a heat-producing load, such as an LED luminaire.
2. Description of the Background
Solid state light emitters including organic, inorganic, and polymer light emitting diodes (LEDs) may be utilized as an energy-efficient alternative to more traditional lighting systems. Many modern lighting applications utilize high power solid state emitters to provide a desired level of brightness. Generally, the lifetime of an LED is inversely related to the operating junction temperature thereof. Therefore, thermal management of the junction temperature is an important design consideration of a luminaire (i.e., a lighting fixture) incorporating one or more LED's. Many solid state luminaires utilize a heat exchanger that dissipates heat into the ambient environment so as to obtain a desired life of the solid state emitters. However, such luminaires can be installed in spaces containing insulation that can prevent the necessary airflow to maintain the junction temperature at or below an upper temperature limit. Because of this possibility, solid state luminaires typically are provided with a thermal protection circuit that disconnects the LEDs from a power source when a sensed temperature is reached. Often, such a circuit includes a bi-metal switch that opens when the sensed temperature exceeds a threshold and/or a one-time thermal fuse that breaks a circuit connection when a fusing temperature is reached.
LEDs operate more efficiently when powered by a direct current (DC) voltage rather than an alternating current (AC) voltage. Some known solid state luminaires utilize low power DC (i.e., at 60 volts or below). While low power DC solid state luminaires have some inherent advantages (including the ability to control the fixture with low power components and cabling), the use of low power DC can be problematic when designing a thermal protection circuit for the luminaire. Specifically, the lack of AC power at the fixture prevents the use of AC rated RP (thermal overload protection) devices and still meet UL1598 requirements. Also, the cost of the AC RP devices has been increasing of late due to the existence of few manufacturers. An off-the-shelf DC rated UL XAPX2/8 recognized thermal protection bi-metal switch is typically not specified for operation up to 60 VDC because such a device is too large and quite expensive. A UL XCMQ2/8 recognized TCO (thermal cutoff fuse) typically has a temperature difference between the fusing and the holding temperatures (i.e., the maximum normal operating temperature) that renders the device impossible to use with typical luminaire maximum operating temperatures.